The present invention relates to a process and an apparatus for activating carbon-containing material.
1. Field of the Invention
In recent years, activated charcoal, due to its properties as an adsorbent material with inert characteristics, has been used increasingly in the most disparate fields, such as for example:
in the processing of potable, process and waste water, where activated charcoal effectively removes odors and tastes as well as residues of colors and dissolved organic substances; PA1 in the purification of air and gases, such as for example in conditioning systems, in sewage systems, and in chemical processes; PA1 in the preparation of filters for masks, hoods, cigarettes, etc.; PA1 in the food industry, in processes for the decolorization of sugar, glucose, vegetable oils, fermenting alcoholic beverages, juices, etc.; PA1 in the pharmaceutical industry, to purify raw materials and intermediate compounds; PA1 in the chemical industry, to purify colors and plasticizers, organic acids, for galvanic deposition baths, in recovering gold from residues of mining processes, in catalysis, both as a medium (hydrogenation, desulfurization, vinyl chloride synthesis) and as an actual catalyst (phosgene, vinyl acetate, etc.); PA1 for use as a medicine, on its own or in association with antiseptics, digestive ferments, etc., in the preparation of tablets or capsules.
Its many uses, combined with the increasingly strict limits set by new statutory provisions in the field of pollution, have caused a considerable increase in the use of activated charcoal. One should also add that processing with activated charcoal is often cheaper than other purifying systems, such as thermal or catalytic reheating, scrubbing, or other adsorption techniques, when the level of the impurities is less than a few hundred parts per million.
2. Description of the Related Art
Activated charcoal is obtained by activating carbon-based material derived essentially from bituminous coals, peat, coconut shells, wood, sawdust, etc. by means of activation processes which are designated as chemical or physical.
Chemical activation uses the particular action of some inorganic compounds (activating agents) during the carbonization step, which is performed at temperatures of 400.degree. to 600.degree. C. The material is initially impregnated with appropriate chemicals which, when heated, release oxidizing gases that degrade the organic molecules. The main known activating agents are zinc chloride and phosphoric acid; positive results have been obtained by using sulfates or phosphates of alkaline metals, potassium thiocyanide, and manganese sulfide. Chemical activation is about to be abandoned since the step for the removal of the activating agent entails considerable ecological problems.
Physical or thermal activation is performed with gaseous activating agents that selectively oxidize the carbon-containing material. The treatment, which is usually preceded by carbonization of the raw material so as to reduce its content of volatile substances, uses air, water vapor or carbon dioxide. During the activation step, part of the carbon is burned, consequently increasing porosity.
Oxidation with air is performed at low temperatures, but the exothermic nature of the reaction entails difficulty in managing it; it is used to produce charcoals having a low level of activity. Conversely, since treatment with CO.sub.2 or H.sub.2 O is endothermic, it is easy to control and thus more widely used despite its high process temperatures.
Presently, activated charcoal is almost entirely produced with the thermal process, using water vapor as a reagent. Thermal activation is performed in reactors normally used in gas-solid reactions in the chemical industry in general. Among these reactors, those currently most used are multiple-hearth furnaces, tubular rotary furnaces and fluid-bed furnaces.
Multiple-hearth furnaces are those most widely used to activate charcoal and are constituted by a metal tower which is internally lined with refractory material and in which various tables are arranged. A shaft rotates at the vertical axis of the tower and has rotating arms fixed thereto; said arms have inclined vanes which have the purpose of moving the material. The charcoal, which is loaded from above, encounters the steam, which is supplied from the lower part, and falls through holes onto the underlying tables since it is moved by the vanes. These reactors have the advantage of well-established technology and high reliability, but they have several problems, including poor efficiency in reaction, which is linked to the particular type of contact between the phases occurring in this reaction. Very poor contact leads to long permanence times, with consequent large volumes and low specific productivity. Furthermore, apparatuses using multiple-table furnaces are very large and complex and therefore require large investments and onerous maintenance operations.
Rotary tubular reactors are instead essentially constituted by a rotating cylinder, at the ends whereof the charcoal and the steam are loaded in countercurrent. These apparatuses are simple but have the drawback that they do not provide good contact between the reagents, and this leads most of all to low productivity.
Fluid-bed furnaces have been adapted to this production in recent years. These furnaces are internally lined with refractory material, and a porous plate is arranged in the lower portion of the furnace; the charcoal is fed onto said plate and the steam passes therethrough. The bed, during agitation, practically doubles its height, providing excellent contact between the phases. This type of apparatus offers, among its many advantages, considerable temperature uniformity, highly effective gas-solid exchange and therefore low permanence times and considerable potential. On the other hand, the fluid bed has, among its disadvantages, the fact that it fragments the product, that it creates considerable erosion problems inside the reactor, and that it furthermore requires high investment costs.
In addition to the above listed specific disadvantages, these known types of apparatus for the thermal activation of charcoal have the drawback of working by direct heat exchange. The hot fumes that must sustain the endothermic nature of the reaction in fact strike the charcoal directly. Excellent heat exchange is obtained in this manner, but on the other hand there is the problem of high specific consumption of charcoal, due to its partial combustion with any excess oxygen contained in the hot fumes. Furthermore, there is a considerable expenditure of energy, since it is necessary to have a reheat unit downstream of the furnace, where the process gases must be treated in order to provide the environmental protection conditions required by applicable statutory provisions.